Inhibition method of stress corrosion cracking of nuclear steam generator tubes by lanthanum boride

ABSTRACT

The present invention relates to a method for inhibiting stress corrosion cracking occurring on steam generator tubes in the secondary side of nuclear power plants and an inhibitor therefor. The method comprises supplying lanthanum boride as an inhibitor for stress corrosion cracking into the secondary feed water of the nuclear power plants. The method according to the present invention improves the resistance of stress corrosion cracking ten times or higher than no inhibitor, and five times or higher than the conventional inhibitor. In particular, the method according to the present invention shows a superior inhibiting effect in the lead-contaminated environment such that the complete inhibition of stress corrosion cracking can be achieved. As thus, the method can reduce the unexpected stop of operating nuclear power plants caused by the stress corrosion cracking on steam generator tubes, thereby reducing the cost required for maintaining and repairing the tubes.

FIELD OF THE INVENTION

[0001] The present invention relates to a method for inhibiting stresscorrosion cracking occurring on steam generator tubes in the secondaryside of nuclear power plants (NPPs) and an inhibitor therefor.

BACKGROUND OF THE INVENTION

[0002] It was reported that Ni alloys, especially, ones used as steamgenerator tubes in nuclear power plants, frequently suffered fromintergranular corrosion and stress corrosion cracking in theenvironment. The intergranular corrosion and stress corrosion crackingoccurring on steam generator tubes in NPPs may result in the leakage ofthe primary cooling water into the secondary side, unexpected stop ofoperating NPPs, and high cost for inspecting and repairing the crackedtubes. Therefore, the development of an inhibitor and a method forinhibiting the intergranular corrosion and stress corrosion crackingoccurring on steam generator tubes in NPPs is urgently demanded.

[0003] Up to now, boric acid has been used as an inhibitor for stresscorrosion cracking. However, it was proved not to be an effectiveinhibitor in inhibiting the intergranular corrosion and stress corrosioncracking on steam generator tubes in NPPs.

[0004] Recently, titanium oxide as an inhibitor, which introduces intothe high temperature and high alkali feed water so as to inhibit stresscorrosion cracking, was reported. However, quantitative analysis of itseffectiveness in inhibiting the intergranular corrosion and stresscorrosion cracking has not been performed yet.

[0005] Furthermore, an inhibitor or a method for inhibiting stresscorrosion cracking occurring on steam generator tubes in environmentscontaminated with at least one lead compound such as lead oxide, leadchloride, lead sulfide which were known to accelerate stress corrosioncracking has not been developed yet.

SUMMARY OF THE INVENTION

[0006] Leading to the present invention, the intensive and thoroughresearch on an inhibitor or a method for inhibiting stress corrosioncracking occurring on steam generator tubes in the secondary side ofNPPs carried out by the present inventors aiming to avoid the problemsencountered in the prior arts, and resulted in the finding that thepowerful inhibition of intergranular corrosion and stress corrosioncracking occurring on steam generator tubes, compared with no inhibitoror the conventional inhibitors such as boric acid and titanium oxide,can be achieved by supplying the lanthanum boride as an inhibitor intothe secondary side feed water of NPPs.

[0007] Therefore, it is an object of the present invention to provide anovel inhibitor in order to overcome the problems caused by theintergranular corrosion and stress corrosion cracking occurring on steamgenerator tubes in the secondary side of the NPPs.

[0008] It is another object of the present invention to providelanthanum boride as an inhibitor for inhibiting intergranular corrosionand stress corrosion cracking occurring on steam generator tubes in thesecondary side of NPPs.

[0009] And, it is a further object of the present invention to provide amethod for inhibiting the intergranular corrosion and stress corrosioncracking occurring on steam generator tubes in the secondary side ofNPPs, which comprises supplying the lanthanum boride as an inhibitorinto the secondary feed water to form chromium-enriched oxide film onthe surface of steam generator tubes.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010]FIG. 1 is a graph showing the relative inhibition effectiveness onstress corrosion cracking in a caustic solution containing no inhibitor(“reference”), a conventional inhibitor (“H₃BO₃”), a conventionalinhibitor (“TiO₂”), and an inhibitor (“LaB₆”) according to the presentinvention.

[0011]FIG. 2 is a graph showing the depth profiles of the main alloyingelements in the film formed at the crack tip of the steam generatortubes in the caustic solution, wherein no inhibitor is added.

[0012]FIG. 3 a graph showing the depth profiles of the main alloyingelement and Ti in the film formed at the crack tip of steam generatortubes in the caustic solution, wherein a conventional inhibitor, thetitanium oxide, is added.

[0013]FIG. 4 a graph showing the depth profiles of the main alloyingelements in the film formed at the crack tip of steam generator tubes inthe caustic solution, wherein the lanthanum boride according to thepresent invention is added.

[0014]FIG. 5 is a graph showing the electrochemical impedance datameasured in the solution with and without lanthanum boride according tothe present invention.

[0015]FIG. 6 is a graph showing the relative inhibiting effect on stresscorrosion cracking in the lead oxide-contaminated caustic environment.

DETAILED DESCRIPTION OF THE INVENTION

[0016] The present inventors discovered that the lanthanum boride couldsignificantly increase the resistance of steam generator tube tointergranular corrosion and stress corrosion cracking in hightemperature caustic environments with and without lead compounds.

[0017] The lanthanum boride in an amount of from about 0.1 ppb to 1,000ppm as an inhibitor for stress corrosion cracking is preferably addedinto the secondary feed water. More preferably is from 1 ppb to 100 ppm.

[0018] According to the embodiment of the present invention, thelanthanum boride improves the resistance to intergranular corrosion andstress corrosion cracking ten times or higher than no inhibitor, andfive times or higher than the conventional inhibitor, titanium oxide.This is clearly shown in FIG. 1.

[0019] Recently, it was reported that the lead compounds such as leadoxide, lead chloride, lead sulfide accelerate stress corrosion crackingoccurring on steam generator tubes in the secondary side of NPPs.According to another embodiment of the present invention, the inhibitoralso showed a very powerful inhibition of stress corrosion cracking inthe lead-contaminated caustic environment. This is shown in FIG. 6.

[0020] The present invention also relates to a method for inhibitingintergranular corrosion and stress corrosion cracking occurring on steamgenerator tubes in the secondary side of NPPs. The method comprisessupplying the lanthanum boride as an inhibitor for stress corrosioncracking into the secondary feed water of NPPs. More specifically, themethod comprises supplying the lanthanum boride in the amount rangingfrom 0.1 ppb to 1,000 ppm into the secondary feed water of NPPs. Theroom temperature pH of the secondary feed water is generally varied from5.0 to 9.5; the temperature is lower than 330° C., more preferably,150-315° C.

[0021] The lanthanum boride added to the secondary feed water has aprotective film formed on the surface of steam generator tubes. Theprotective film can be formed by circulating the secondary feed waterunder the normal operating conditions, or by stopping it for from 0.5 to240 hours. Particularly, the protective film formed in the solutioncontaining the lanthanum boride has been found to show a powerfulinhibiting effect on the stress corrosion cracking occurring on steamgenerator tubes in the environment contaminated with the lead compounds.

[0022] The application of the preferred embodiments of the presentinvention is best understood with examples and the accompanyingdrawings.

[0023]FIG. 1 is a graph showing the relative inhibiting effects onstress corrosion cracking of Alloy 600 steam generator tubes for noinhibitor (“reference”), boric acid (“H₃BO₃”), titanium oxide (“TiO₂”),and lanthanum boride (“LaB₆”) according to present invention,respectively.

[0024] The experiments were carried in 10% NaOH solution at 315° C.,because the stress corrosion cracking in the secondary side of steamgenerator tubes was frequently observed to occur in a high-alkalienvironment during the operation of the steam generator.

[0025] The specimens for the stress corrosion cracking test werefabricated as C-ring and stressed until their outer diameter wasdeflected by 1.5 mm. To accelerate the stress corrosion cracking rate,the specimens were polarized at a potential of 150 mV above thecorrosion potential. The amount of the inhibitors used was 4 g/L. After5 days, the percentage of the stress corrosion crack depth relative tothe thickness of the specimen was measured and the result thereof wasshown in FIG. 1. These values were obtained by averaging the maximumcrack depths developed from the different three specimens. As shown inFIG. 1, the lanthanum boride according to the present invention hashighly improved the resistance to stress corrosion cracking ten times orhigher than no inhibitor (which is denoted as “reference”), and fivetimes or higher than the conventional inhibitor, titanium oxide. Inaddition, boric acid was not effective to inhibit cracking under thissevere environmental condition.

[0026]FIG. 2 is a graph showing the depth profiles of the main alloyingelements, measured with a scanning Auger spectroscope, in the surfacefilm formed at the crack tip of the steam generator tubes in the causticsolution, wherein no inhibitor is added, and FIGS. 3 and 4 are for thetitanium oxide and for the lanthanum boride, respectively.

[0027] The depth profiles of the elements in the film were obtained bysputtering with argon. The compositions at the left region, as shown inFIGS. 2, 3 and 4, represent the compositions at the outer layer of thefilm

[0028] As shown in FIG. 2, with no inhibitor, no chromium was observedin the outer layer of the film and thus the film was found to besignificantly chromium-depleted.

[0029] With the conventional inhibitor, titanium oxide, the extent andthe depth of the chromium depletion in the film were found to be lesssignificant than that with no inhibitor (FIG. 3). The chromium depletionin the films on tube material is due to the selective dissolution ofchromium. Consequently, the films become porous and non-protective,resulting in susceptible to stress corrosion cracking.

[0030] To the contrary, when the lanthanum boride was added as aninhibitor for stress corrosion cracking, chromium-depleted region in thefilm was not observed. Further, it was found that chromium was enrichedin the outer layer of the film.

[0031] It was widely acknowledged that chromium oxides play an importantrole in improving the resistance to localized corrosion, such aspitting, stress corrosion cracking, intergranular attack, etc.

[0032] As thus, the lanthanum boride according to the present inventioncan highly improve the resistance to stress corrosion cracking byforming a protective and chromium-enriched oxide film. This is shown inFIG. 4. That is, the inhibitor and the inhibiting method according tothe present invention will exhibit the improved inhibition of stresscorrosion cracking, which is distinguished from the conventionalinhibitors or methods.

[0033] Electrochemical impedance measurements were made at the corrosionpotential in 10% NaOH solution with and without lanthanum boride at 315°C. FIG. 5 shows Nyquist plot of the impedance spectra. The frequencyresponse curves in the solution containing no inhibitor showed the extratime constant in low frequency range. However, Nyquist curve obtained inthe lanthanum containing solution showed only one capacitive loop. Thisresult indicates that the addition of lanthanum boride decreased thecorrosion rate of the tube material, especially selective dissolution ofchromium, which can be well rationalized by the result obtained in FIG.3.

[0034]FIG. 6 is a graph showing the inhibiting effect of the lanthanumboride on stress corrosion cracking of Alloy 600 steam generator tubesin lead oxide-contaminated environment.

[0035] The experiment was conducted in 10% NaOH aqueous solutioncontaining 5,000 ppm of PbO at 315° C.

[0036] The specimens for the stress corrosion cracking tests werefabricated as C-ring and stressed until their outer diameter was reducedby 1.5 mm. To accelerate the stress corrosion cracking rate, thespecimens were polarized at a potential of 150 mV above the corrosionpotential. The amount of the inhibitors used was 4 g/L for the titaniumoxide and was 1 g/L for the lanthanum boride. After 5 days, thepercentage of the stress corrosion crack depth relative to the thicknessof the specimens was measured and the result thereof is shown in FIG. 6.

[0037] The depth of the crack formed in the caustic solution containingno inhibitor was measured to be 70% of the thickness of the specimens,and 40% in the solution with the titanium oxides. However, with thelanthanum boride according to the present invention, no cracking wasformed.

[0038] Therefore, the inhibitor, lanthanum boride, according to thepresent invention has a superior inhibiting effect on stress corrosioncracking in the environment contaminated with lead compounds.

[0039] The present invention has been described in an illustrativemanner, and it is to be understood that the terminology used is intendedto be in the nature of description rather than of limitation. Manymodifications and variations of the present invention are possible inlight of the above teachings. Therefore, it is to be understood thatwithin the scope of the appended claims, the invention may be practicedotherwise than as specifically described.

What is claimed is:
 1. An inhibitor supplied into the secondary feedwater to inhibit intergranular corrosion and stress corrosion crackingoccurring on steam generator tubes in nuclear power plants, wherein theinhibitor is lanthanum boride.
 2. The inhibitor as set forth in claim 1,wherein the lanthanum boride is used in an amount of from 0.1 ppb to1,000 ppm.
 3. The inhibitor as set forth in claim 1, wherein thesecondary feed water is contaminated with at least one lead compound. 4.A method for inhibiting intergranular corrosion and stress corrosioncracking occurring on steam generator tubes in the secondary side ofnuclear power plants, comprising the step of supplying the lanthanumboride as an inhibitor for stress corrosion cracking into the secondaryfeed water of nuclear power plants.
 5. The method as set forth in claim4, wherein the lanthanum boride is used in an amount of from 0.1 ppb to1,000 ppm.
 6. The method as set forth in claim 4, wherein the pH of thesecondary feed water at 20° C. is from 5.0 to 9.5 and the temperature islower than 330° C.
 7. The inhibition method as set forth in claim 4,wherein the lanthanum boride-added feed water is circulated to form aprotective film on the surface of the steam generator tubes in nuclearpower plants.
 8. The method as set forth in claim 4, wherein thecirculation of the lanthanum boride-added feed water is stopped for from0.5 to 240 hours to form a protective film on the surface of the steamgenerator tubes in nuclear power plants.
 9. The method as set forth inclaim 4, wherein the secondary feed water is contaminated with at leastone lead compound.
 10. The method as set forth in claim 9, the leadcompound is selected from the group consisting of lead oxide, leadchloride and lead sulfide.